1. Field of the Invention
The present invention relates to a process of producing ferromagnetic materials.
2. Description of the Prior Art
Maghemite (gamma-Fe.sub.2 O.sub.3) has hitherto been widely used for magnetic recording media such as audio magnetic tapes, video magnetic tapes, memory tapes, magnetic sheets, magnetic cards, etc. Also, cobalt ferrite prepared by incorporating cobalt, etc., in maghemite has been used as a high-coercivity material.
These magnetic recording media can be used in a wide technical field for recording and reproducing electrical or magnetic signals and, in particular, recording signals of short wave lengths at high density has become important recently. For such use ferromagnetic materials are required to show magnetic recording characteristics suitable for high-density recording, such as a high coercivity and a high residual magnetic flux density. Furthermore, ferromagnetic materials in which magnetic signals are not demagnetized by pressure, e.g., due to repeated reproduction, have been required.
As one example of ferromagnetic materials showing low demagnetization which are suitable for high-density recording, there are Berthollide-type iron oxide particles containing a transition element.
Such magnetic materials have a variable oxidation ratio and comprise compounds shown by the following formula EQU MyFe.sub.l.sub.+y O.sub.x
wherein M represents a transition element or at least one element selected from the group consisting of the elements belonging to groups IIIb, IVb, VIb, VIIb, VIII, Ib, and IIb of the Periodic Table, y is a number between 0.001 and 0.25, and x is a number between 1.33 and 1.50.
Such high-power ferromagnetic materials are commercially available as, for example, Beridox (produced by Fuji Photo Film Co.).
The transition element improves the magnetic properties of the ferromagnetic iron oxide particles. The transition element is usually added to ferromagnetic iron oxide particles or iron oxyhydroxide particles. The inventors earlier found that the more uniformly the addition of the transition element to the Berthollide type ferromagnetic iron oxide particles is carried out, the more improved the erasure characteristics and the print-through characteristics of the resulting magnetic recording medium (characteristics important once signals are recorded on a magnetic recording medium and one wishes to re-record thereon other signals). Print-through or magnetic reprint is the phenomenon where signals recorded on a magnetic recording medium are printed or transferred onto adjacent magnetic recording medium by the magnetic field originating from the signals, and it is important that a magnetic recording medium shows such a phenomenon to a low degree.
In the case of incorporating a transition element in such ferromagnetic iron oxide particles, iron oxide particles or iron oxyhydroxide particles are dispersed in water and the first of two solutions which cause the formation of a precipitated salt of the transition element by reaction when they are mixed together is added to the dispersion followed by mixing to provide a slurry. Then, the second solution is added to the slurry to form a precipitate of the salt of the transition element on the surface of the iron oxide particles or the iron oxyhydroxide particles. The particles are then recovered by filtration, dried, and subjected to a heating to fix the transition element in the particles or on the surfaces of the particles. Such processes are described in detail in, for example, Japanese Patent Publication 6538/1966, and Japanese Patent Application Laid Open Nos. 76,097/73 and 22,707/72.
A particularly important matter in such operations is that the precipitate containing the transition element be fixed uniformly on or in the iron oxide particles or oxyhydroxide particles.
Chromium dioxide (CrO.sub.2) is also known as a ferromagnetic substance suitable for magnetic recording medium. That is, a magnetic recording medium having a magnetic recording layer composed of an organic binder and fine chromium dioxide particles dispersed in the binder is superior to a conventional magnetic recording medium having a maghemite (gamma-Fe.sub.2 O.sub.3) per se magnetic recording layer at the following points:
1. Since chromium dioxide particles have good dispersibility in a binder, a magnetic recording medium prepared using chromium dioxide particles has a smooth surface.
2. Chromium dioxide particles show high coercivity, e.g., 400-700 oersteds, and thus such a magnetic recording medium is suitable for high-density recording.
3. Chromium dioxide particles possess a high squareness ratio (Br/Bm), e.g., 0.85-0.9, due to their good dispersibility in a binder.
4. Since chromium dioxide particles possess a high magnetic flux density, it is possible to reduce the thickness of the magnetic recording layer.
5. Chromium dioxide particles also possess a comparatively low Curie point, which makes it possible to practice thermal duplication of recordings.
Processes of producing chromium dioxide particles suitable for magnetic recording are described in, for example, U.S. Pat. Nos. 3,449,073; 3,278,263; 3,696,039; 3,371,043; 3,687,851; 3,600,315; 3,600,314; 3,778,373; etc. Chromium dioxide particles prepared by these processes can be used in the process of this invention.
The magnetic properties, in particular, the coercivity of chromium dioxide, can be changed using modifiers during or after the production thereof. Techniques involving such modifiers are described in, for example, U.S. Pat. Nos. 3,574,115; 3,687,851; 3,583,917; 3,726,714; 2,886,365; 2,923,683, etc. The chromium dioxide particles prepared by these processes are also suitable as the raw materials treated by the process of this invention.
However, it has become clear that chromium dioxide has various faults. That is, when a magnetic recording medium having a chromium dioxide layer is stored under high temperature humidity conditions, saturation magnetization (.sigma.S) is reduced. Furthermore, chromium dioxide is partially decomposed and thus chromium ions tend to dissolve out.
It is known, to improve these faults, to stabilize chromium dioxide particles by coating the surfaces of the particles with a proper material, e.g., as described in U.S. Pat. Nos. 3,687,726; 3,512,930; 3,686,031; 3,586,630; 3,585,141; 3,736,181; 3,769,087; etc. In these processes chromium dioxide particles are stabilized by coating the surfaces of the particles with an inorganic compound such as CrOOH, Sb.sub.2 O.sub.3, As.sub.2 O.sub.3, Sb.sub.2 S.sub.3, As.sub.2 S.sub.3, Al.sub.2 O.sub.3, a phosphate, aluminum hydroxide, etc., or by treating the surfaces of the particles with an organic phosphorus compound or an organic cyclic amine.
The inventors investigated these processes and, as a result, discovered that these processes cannot provide a sufficient effect for the desired purpose, i.e., the chromium dioxide particles are apt to become coarser and, further, their dispersibility is apt to become lower. In this case, the inherent excellent surface properties of a chromium dioxide magnetic recording medium are likely to be lost. Further, these conventional stabilization processes require delicate operation techniques, and if the stabilization treatment is prolonged, magnetization is apt to be reduced. It has further been found that when chromium dioxide particles subjected to such a stabilization treatment are kneaded with a binder, the coated layers on the particle surfaces are gradually destroyed. This is considered to be caused by poor adhesivity between the coated layer and the inside chromium dioxide particle.
As the results of investigations on the aforesaid points, the inventors have succeeded in discovering a new process of producing and/or stabiliing ferromagnetic materials.